Process flows in modern semiconductor device fabrication lines utilize a multitude of fabrication steps performed by a series of semiconductor fabrication tools. In some cases, process flows may contain more than 800 process steps in a re-entrant flow among a set of semiconductor device fabrication line tools including over one thousand equipment units of over one hundred different types. Wafers in a process flow are typically moved through process steps in sealable containers, or front opening unified pods (FOUPs), by an automated material handling system (AMHS). Each semiconductor device fabrication line tool contains a standardized equipment front end module (EFEM), alternatively a load port, for receiving the sealable containers. Once a semiconductor device fabrication line tool has received a sealable container, the sealable container is opened in a controlled environment, processed, and returned to the FOUP for pickup by the AMHS. In this regard, the sealable container provides a clean and protective environment for the storage and transfer of wafers in a process flow.
It is generally desirable to reduce idle time, or starvation, of semiconductor device fabrication line tools associated with a deficiency of sealable containers available at the tool load port to be processed. However, the processing time of each semiconductor device fabrication line tool and the delivery time of sealable containers between semiconductor device fabrication line tools by the AMHS may vary, necessitating advanced process flow optimization systems. Typical semiconductor device fabrication line tools contain two to four load ports to serve as temporary storage for incoming or outgoing sealable containers. However, the additional load parts may not be sufficient to eliminate starvation, particularly for high throughput semiconductor device fabrication line tools. Further, practical considerations typically limit the number of load ports for a given semiconductor device fabrication line tool. For example, increasing the number of load ports consumes valuable space on a process line, increases travel requirements for wafer-handling systems (e.g. an AMHS), and increases the size of environmental enclosures surrounding the semiconductor device fabrication line tools, all of which increase operational expenses and increase maintenance requirements. Therefore, it would be desirable to provide a system and method for curing defects such as those identified above.